My husband and I booked an all day tour with Blackwater Tours. We never knew that Monument Valley was vast, and had a long history of it's own. Our tour guide "Willie", was very insightful, and fun to be with. We spent the whole day driving into canyons, back roads, and over sand dunes.
We did take a trip into Monument Valley the day before, but with another tour company. It was uninviting, commercialized, timed, and crowded.
I recommend booking with a local tour company first, don't judge a book by it's cover.